Real-time monitoring of fatigue cracks and stress corrosion cracks in helicopter rotor heads is a difficult task. Such cracks are a significant problem for rotor systems on helicopters. Class "A" Helicopter mishaps have risen at an alarming rate in the last decade. A class A mishap is defined as the loss of a vehicle (i.e., a rotor craft). From 1980 to 1990, almost half of the mishaps were due to class A failures. Recent British studies performed by the Helicopter Air worthiness Review Panel (HARP) indicated that 33 percent of the accident mishaps were caused by a main rotor failure leading to loss of life and aircraft. An additional 25 percent of mishaps due to main rotor problems caused the aircraft to be ditched at sea.
The inventor and his employer have been studying the effects of metal fatigue on commercial transport and military aircraft. Working directly with a major airline and as a major subcontractor in a U.S. Air Force smart metallic structures program, the inventor has learned that real-time structural health monitoring for aircraft involves an additional dimension of complexity beyond the conventional nondestructive evaluation (NDE) techniques for detecting structural integrity problems such as fatigue cracking and hidden corrosion. Structural integrity inspection is typically localized to rotor head hub assemblies, bearings, connection linkages, and tie bars.
In a typical rotor, each blade has three distinct bearings (commonly called hinges) at its hub end, allowing movement in the feathering, flap, and lead/lag axis. The hinges may incorporate metal ball-races, or an elastometric bearing made of synthetic rubber to minimize rotor head vibration effects. Fatigue cracks occur in highly loaded rotor head components. These rotor components are susceptible to corrosion cracking in such environments as moist sea air, sea water and acid rain. The rotor head components experiencing fatigue cracks include the main rotor hub, the connecting link, the pitch shaft, the tie bar and pin, the pitch housing, the lag dampers, the forward and aft rotor drive shafts, and the blade fittings. Fatigue cracks occur in the ball-races of the main rotor hub, the rotor hub spline area, the pivot area of connecting links, the individual laminates of the tie bar assembly, the tie bar pin, the inspection access holes on the aft rotor drive shaft, damper attachment points, and the blade fitting.
Several NDE methods are available to detect metallic-related fatigue cracks, but each method has one or more significant technical limitations. These detection methods include visual, tap test, ultrasonic, eddy current, and x-ray radiography. Visual inspection is appropriate for checking surface conditions such as cracks in the main hub body or general surface corrosion but is not effective for detecting cracks within the ball-races of the main hub assembly. A low-frequency eddy current can detect cracks in rotor system components but requires an extensively trained NDE technician to properly position the eddy current probes and interpret test results. X-ray radiography can be used but requires special equipment, and limits general maintenance crew access to the aircraft while the X-ray testing is being performed. Each of these NDE methods also has two significant drawbacks. First, some mechanical disassembly of the helicopter rotor is required which increases operational costs and limits flight availability time. Second, such methods are not real-time health monitoring solutions which can provide an early warning indication of a structural crack initiation or crack propagation event.